The present invention relates to rotary magnetic position sensors used to measure angular displacements, and more particularly to permanent magnets used therein. Still more particularly, the present invention relates to pole differentiated permanent magnets used in the aforesaid application.
The use of magnetoresistors (MRs) and Hall devices as position sensors is well known in the art. For example, a magnetically biased differential MR sensor may be used, for example, to sense angular position of a rotating toothed wheel.
Position sensors with digital outputs provide discrete position information only whereas an analog position sensor can provide both position information and outputs that can be used to drive an electric motor or other similar electromechanical devices. Many of these devices are driven by sinusoidal excitations as a function of position. Consequently, an analog position sensor having an output that varies sinusoidally with position could be used to generate absolute angular positions as, for example, an electrical power steering system to measure the angle of rotation of the steering wheel, and/or reference signals to produce the desired sinusoidal phase drive currents and voltages to drive electric motors and other similar electromechanical devices. Applications may further include throttle position sensors, pedal position sensors in drive-by-wire systems, body height position sensors for automotive suspension systems, a 3-phase sine generator for brushless motors, a sine/cosine resolver for servo motors, etc.
The operational principle of an angle encoder sensor is based upon the property of Hall plates and semiconductor magnetoresistors, collectively referred to herein as magnetosensitive devices, to sense only the normal component of the magnetic field passing through them. Consequently, if a constant and uniform magnetic field is rotated in the plane perpendicular to the surface of a magnetosensitive device, the output signal will vary as the cosine of the angle between the direction of the incident magnetic field lines and the line normal to the surface of the device. It is preferred in this regard, that the magnetosensitive device be linear in its response to change in direction of the incident magnetic field, such as that provided by Hall plates; however, magnetoresistors operating in their linear region can also be used. In addition, operation over any ambient temperature range may require temperature compensated magnetosensitive devices. Also, it should be noted that included by the term xe2x80x9cmagnetosensitive devicesxe2x80x9d are ferromagnetic magnetoresistors, including giant magnetoresistor (GMR) sensors, which can also be used although these are less preferred because their resistance versus magnetic flux density saturates at a relatively low level compared with Hall sensors that do not saturate.
The assembly of rotary magnetic position sensors involves two permanent magnets in diametric opposition to each other, wherein the facing poles are of opposite polarity so as to thereby provide a generally uniform magnetic field across the space occupied by the magnetically sensitive sensor device. One technique used in the assembly is to insert non-magnetized blocks of magnetizable material, over-mold the magnet carrying component, then magnetize the blocks by an external magnet to thereby convert the blocks into permanent magnets having facing poles with opposed polarity.
Problematically, however, if pre-magnetized permanent magnets are placed into a mold tool for forming the magnet carrying component, and the magnets are identical, there could be a chance that like poles could face each other, rather than unlike poles facing each other.
Accordingly, what remains needed is a compact inexpensive contactless position sensor having pole differentiated permanent magnets, wherein the magnets provide sinusoidally varying output suitable for specialized sensing schemes.
The present invention is a rotary magnetic position sensor (also referred to as an analog analog encoder) having a pair of pole differentiated permanent magnets, wherein rotation of a magnetic field relative to a magnetosensitive device provides a varying output of the magnetosensitive device that varies sinusoidally with the angle of relative rotation.
The rotary magnetic position sensor according to the present invention includes a non-magnetic stator which is preferably cup shaped, having an outer annulus, a bottom disk and a central post connected with the bottom disk and concentrically positioned relative to the outer annulus. Located within the central post is located at least one magnetosensitive device, for example a Hall sensor or a magnetoresistor, the leads of which pass out from the cup. A rotor in the form of a ring captured between the central post and the outer annulus, wherein the ring is rotatable relative to the stator.
A pair of permanent biasing magnets are located in the rotor in mutually diametrically opposed relation, with unlike poles facing each other so as to provide a magnetic field incident upon the at least one magnetosensitive device. In this regard, each magnet has an external asymmetric geometrical feature which defines indexing of the polarity of the magnet. For example, in a pair of wedge shaped permanent magnets, both magnets have an identical smaller area pole face, and an identical larger area opposite pole face, and for both magnets the north pole is at one of the larger or smaller pole face, and the south pole is at the other pole face. As a result, mere visual inspection, or physical contact, allows immediate distinguishment as to whether opposite poles face each other. For example, if the north pole is at the larger area pole face and the south pole is at the smaller area pole face, then for the magnets to have opposed poles facing each other, one facing pole face must be the larger area pole face and the other facing pole face must be the smaller area pole face.
In operation, as the rotor rotates relative to the stator, the angle of the incident magnetic field changes relative to the magnetosensitive device, thereby causing the output from the device to vary sinusoidally with angular position.
Accordingly, it is an object of the present invention to provide a rotary magnetic position sensor having a pair of permanent magnets, wherein the magnets are geometrically pole differentiated.
This, and additional objects, features and benefits of the present invention will become apparent from the following specification.